KVM: allow userspace to adjust kvmclock offset

When we migrate a kvm guest that uses pvclock between two hosts, we may
suffer a large skew. This is because there can be significant differences
between the monotonic clock of the hosts involved. When a new host with
a much larger monotonic time starts running the guest, the view of time
will be significantly impacted.

Situation is much worse when we do the opposite, and migrate to a host with
a smaller monotonic clock.

This proposed ioctl will allow userspace to inform us what is the monotonic
clock value in the source host, so we can keep the time skew short, and
more importantly, never goes backwards. Userspace may also need to trigger
the current data, since from the first migration onwards, it won't be
reflected by a simple call to clock_gettime() anymore.

[marcelo: future-proof abi with a flags field]
[jan: fix KVM_GET_CLOCK by clearing flags field instead of checking it]

Signed-off-by: Glauber Costa <glommer@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/include/linux/kvm.h b/include/linux/kvm.h
index b694c1d2f918..6ed1a12ed526 100644
--- a/include/linux/kvm.h
+++ b/include/linux/kvm.h
@@ -439,6 +439,7 @@ struct kvm_ioeventfd {
 #ifdef __KVM_HAVE_XEN_HVM
 #define KVM_CAP_XEN_HVM 38
 #endif
+#define KVM_CAP_ADJUST_CLOCK 39
 
 #ifdef KVM_CAP_IRQ_ROUTING
 
@@ -512,6 +513,12 @@ struct kvm_irqfd {
 	__u8  pad[20];
 };
 
+struct kvm_clock_data {
+	__u64 clock;
+	__u32 flags;
+	__u32 pad[9];
+};
+
 /*
  * ioctls for VM fds
  */
@@ -562,6 +569,9 @@ struct kvm_irqfd {
 #define KVM_SET_BOOT_CPU_ID        _IO(KVMIO, 0x78)
 #define KVM_IOEVENTFD             _IOW(KVMIO, 0x79, struct kvm_ioeventfd)
 #define KVM_XEN_HVM_CONFIG        _IOW(KVMIO, 0x7a, struct kvm_xen_hvm_config)
+#define KVM_SET_CLOCK             _IOW(KVMIO, 0x7b, struct kvm_clock_data)
+#define KVM_GET_CLOCK             _IOR(KVMIO, 0x7c, struct kvm_clock_data)
+
 
 /*
  * ioctls for vcpu fds